Sebastiano Magnanini is believed to have got involved with the Camden drugs underworld.
Photograph: Luke Allen

Weeks before he died, Sebastiano Magnanini excitedly messaged his friends in London. He was coming “back to Blighty”, he announced, and planning to embark on a journalism career. “Hey dude. Whassup. Gonna be back soon!!! Tell me, where is a good course for writing journalism? Let me know. Ciaooo,” he texted a former flatmate.

After all, he had plenty of stories to tell: from being jailed in his 20s for stealing a £1m painting, to his nomadic lifestyle as a tour guide in Cambodia, carpenter in south London and roadie for Prince – Magnanini had done it all. Seba, as he was known to his friends, lived several lifetimes in his 46 years.

It all came to an abrupt end on 24 September. A man and his seven-year-old child found the Italian’s body floating in a quiet stretch of Regent’s Canal in north London. His heavily tattooed corpse had been bound to a shopping trolley and dumped in the water, curled in the foetal position.

Islington tunnel on Regent’s Canal in London. Photograph: David Wilcock/PA

The grim discovery sparked an immediate investigation by Scotland Yard detectives. On Tuesday, three men pleaded guilty to being involved in his death, including two who admitted dumping his body in the canal. Yet no one has been arrested for murder and the investigation is ongoing. His death – officially, at least – remains a mystery.

However, Magnanini’s friends in London and Cambodia have painted a picture of a troubled soul who was fighting drug addiction and had returned to the capital to try get his life back on track. Some believe he died of an overdose after falling back into the Camden drugs underworld, where he was first taken in over 10 years ago.

“He was good at lots of things but he was haunted by the drugs,” said Luke Allen, Magnanini’s former flatmate who had known him since 2004. The Italian suffered a heroin overdose three years ago at Arlington House, the giant hostel for homeless men in Camden, and had received treatment at a nearby drugs clinic for heroin addiction, taking interferon medication to treat hepatitis C. He had gone back and forth between London and Cambodia in an attempt to break the drugs cycle.

Magnanini’s body was found close to a notorious pick-up point for Camden’s drug users. “That’s the lowest rung. He shouldn’t have been there. He had a job. But he’d run out of people who were in contact with that world who wanted to be the go-between for him,” said Allen, who has given police a statement about Magnanini’s death.

When detectives launched the murder investigation, interest quickly turned to the Italian’s past. In 1993, he and two friends stole an 18th-century painting, The Education of the Virgin by Giovanni Battista Tiepolo, worth nearly 2bn lire (then about £1m) from a church in his hometown, Venice.

It was hardly the perfect heist: a Venetian police chief described the thieves as “tragicomic” after taking a break from their getaway to drink beer and smoke joints at a local bar. The case gained Italian media attention – and 20 years later the attention of the British press because the judge, sentencing Magnanini to 18 months in prison, said he could not rule out the possibility of the thieves working for the mafia.

Magnanini’s friends, however, laughed off any mafia connection to his death. They revealed he had been working as a high-end tour guide around the temples of Siem Reap and Phnom Penh in Cambodia, where he stood out for wearing a sharp suit, speaking fluent English, Italian, Spanish and even some Khmer, the official language of Cambodia.

“He had the best reputation. I honestly think he was the best guide ever,” said Benjamin Kremer, a German travel company owner who was Magnanini’s neighbour in Siem Reap. “He had a library of books about the temples and he studied them and he was so into it. You could ask him any question about the history of the temples and he could answer them. It was the only thing he was 100% dedicated to.”

Sebastiano Magnanini with a friend in a bar. Photograph: Liam MacKenzie

While working as a guide in Siem Reap, Magnanini lived near the sculptor Sasha Constable, the daughter of the late painter Richard Constable, both descendants of the famed Suffolk artist John Constable. The pair immediately hit it off, she said, and had neighbourly gatherings at his house which was filled with books, musical instruments, a pool table and two enormous pythons.

“Sebastiano had an amazing zest for life. He was a troubled soul but he had a huge spirit,” Constable said. “Although a lot of people come here for drugs, coming to Cambodia was for him to try and confront his issues and stay away from them because he had a good job here, he built up a good group of friends, he was playing music, so it was almost like he was turning his life around. So it was a massive shock when I heard about how his life ended, as it was something I thought he’d moved away from. It was a very tragic ending.”

In London, Magnanini worked as a carpenter, a trade he was equally passionate about. In February 2014, he landed a job assembling Prince’s stage when the rock star played three nights of concerts at Koko in Camden. It was to be one of the highlights of Magnanini’s adventure-filled life.

He was about to embark on another adventure last summer when he returned to London, keen to start a journalism course before jetting off to India in November. On 3 August, seven weeks before he died, he sent a WhatsApp message to Allen: “Would u give ur house up for a year at £1300 a month? Just a year, for my girl to be near her study.... Or find me something similar?... Love u.”

It was the last time the two would be in contact. Magnanini checked his WhatsApp messages a final time at 3.15pm on 21 September – 24 hours before he was last seen alive – before disappearing from CCTV coverage near King’s Cross train station and Caledonian Road, near the stretch of canal where his body was later found.

Michael Walsh, 41, and Paul Williams, 61, have both pleaded guilty to preventing the lawful burial of a body. Daniel Hastie, 22, has admitted conspiracy to commit fraud by false representation. They will be sentenced at Blackfriars crown court in London on 5 February.